Tufts researchers unpack earthquake triggers

What causes an earthquake to begin? This question drives the latest research from Associate Professor Robert Viesca of the Department of Civil and Environmental Engineering and former Tufts postdoc Federico Ciardo, who explore the mechanics behind these powerful natural phenomena. Published in Proceedings of the Royal Society A, their research sheds light on what causes faults–cracks in the Earth’s crust–to suddenly slip, releasing energy in the form of an earthquake.

Viesca and Ciardo collaborated on the paper titled, “Nonlinear stability analysis of slip in a single degree of freedom elastic system with frictional evolution laws spanning ageing to slip.” In the study, they use a simplified spring-block model to explore how and when a fault becomes unstable in response to external perturbations (disturbance to a system). The model incorporates rate-and-state friction, a phenomenological framework that describes how friction on a fault depends both on how fast the fault is slipping and how long its surfaces have been in contact. While earlier studies focused on two specific ways this friction can evolve—known as the aging and the slip laws—this research investigates the full range of behaviors in between. By doing so, the researchers assess how sensitive fault stability is to the precise form of frictional behavior, and whether faults that appear stable under commonly used friction descriptions can still become unstable when subjected to perturbations, especially if the actual frictional response deviates, even slightly, from a common description, the ageing law. 

Because the spring-block model serves as an idealized representation of how real faults behave under stress, the study also provides insights into natural fault systems. It shows that even subtle changes in frictional behavior may increase the likelihood of a slip instability (or earthquake). The analysis also helps determine the smallest possible fault or fault patch size that can host such an instability. While classical models suggest a well-defined critical length for slip instability, this study finds that such a threshold may vanish when the frictional behavior deviates from the aging law—meaning that instabilities can arise over smaller regions of a fault, provided the perturbation is strong enough. The critical size depends on the fault’s geometry and configuration, its frictional properties, the loading conditions, as well as the properties of the surrounding rock.

Consistent with both laboratory earthquake experiments and more complex continuum models, these results suggest that fault zones may be far more susceptible to slip instability than previously thought.

Viesca's broader research focuses on theoretical mechanics in earth sciences and engineering. His latest paper builds on previous work, “Frictional state evolution laws and the non-linear nucleation of dynamic shear rupture”, published in 2023’s Journal of the Mechanics and Physics of Solids. Viesca’s other research interests include asymptotic methods in fracture mechanics,, fault rupture initiation and propagation, and fluid-induced seismicity. 

Learn more about Associate Professor Robert Viesca, and former postdoc Federico Ciardo, Assistant Professor at Northwestern University since September 2024.